This invention relates to steel reinforcing elements, such as wire, cord, cable and the like, for elastomeric composite materials. More particularly, the invention relates to steel wire for being embedded in a rubber material vulcanizable with sulphur to obtain a reinforced rubber article, such as e.g. a vehicle tire. The wire is covered with a thin coating of brass to improve the bonding with the rubber compound during the vulcanization process. The present invention also extends to reinforcing cable and cord made from the coated steel wires, as well as rubber products reinforced therewith, and particularly pneumatic tires for vehicles.
Steel wires and like products are now generally used for strengthening a variety of rubber products, including tires, conveyor belts, hoses and the like. Such a reinforcing element can be a monofilament, but it is normally prepared from several filaments which are twisted together to form a strand. The strand of filaments can be further assembled to form a steel tire cord, a belt cord, a cable, a weft of wires and/or cords and other combinations. Such reinforcing elements are usually comprised of brass plated high-carbon steel wire having a diameter of up to 2 mm, mostly from 0.05 to 0,50 mm for tire cord, and may have a carbon content of 0.40 to 1.40%, and preferably 0.60 to 1.0% C.
In the automobile tire manufacturing industry, for instance, large scale use is made of the reinforcement of rubber with steel cord made from a number of 0.70% carbon steel wires in a diameter range of 0.10 to 0.40 mm, the wires being covered with a thin rubber adherent brass coating. These steel cords are first brought into contact with unvulcanized rubber. Subsequently the rubber is subjected to a vulcanization treatment during which the adhesion between rubber and brass is effected.
It is generally known that a brass composition, containing from 55 to 75% Cu (the remainder being essentially zinc), and preferably from 60 to 72% Cu, is suitable for attaining a reasonable adhesion level. However, in order for rubber articles which are reinforced with brass-coated steel wire or cord to function effectively, it is imperative that good adhesion between the rubber and the brass be maintained during the lifetime of the article and under all circumstances.
In this respect, it is recognized that the conditions to which automobile tires, for example, may be subjected are so demanding (e.g. high dynamic loads, humidity, salt corrosion, heat ageing, etc.) that the required adhesion retention between the steel cord and the rubber is not always satisfactorily met by known brass-coated cords.
With regard to the adhesion between a brass-coated wire and rubber, a distinction should be made between the initial adhesion level and adhesion strength after ageing, i.e. after humidity, corrosion and temperature effects occurring during the running life of a tire, for instance.
Initial adhesion is, in general, determined by measuring the force (by means of a tensile tester) required to pull out the rubberized cords, the cords having been bonded by vulcanization to a given rubber. The pull-out force is expressed in Newtons (this being the test procedure for the adhesion of tire cord according to ASTM D 2229-80).
The effect of ageing in service can be simulated by subjecting the vulcanized cord samples to an ageing treatment in a moist atmosphere or in a steam atmosphere at a prescribed temperature for a variable time. After this treatment the actual adhesion level can be evaluated by measuring the bond strength by means of a cord pull-out test carried out on a tensile tester, as described above, or by measuring the rubber coverage of the cords which have been separated from the rubber sample (either complete or partial separation depending on the type of test).
For the purpose of illustrating the effect of ageing on the residual adhesion strength, we chose the following test conditions: namely, in a humidity aged adhesion test, a vulcanized rubber cord sample was treated for a variable time at 70.degree. C. in an atmosphere of 95% relative humidity, and in a steam ageing test, a vulcanized cord sample was kept in a steam atmosphere at 120.degree. C. After these ageing treatments the adhesion force was measured on the tensile tester (cord pull-out test according to ASTM 2229-80).
The quality of the adhesion of the reinforcing elements to the rubber is also indicated by the degree of rubber coverage. This is the amount of rubber left on the reinforcing wire or cord after it is pulled out or otherwise separated from the vulcanized rubber matrix, such as e.g. by a peel test or by a strip separation test. We used the strip test. The amount of rubber-coverage was then evaluated visually and was expressed as an appearance ratio index on a scale ranging from 0 to 10, whereby index 0 refers to zero coverage and index 10 refers to full rubber coverage, or on a scale of from 1 to 5 (under 5 referring to 100% coverage and index 1 referring to less than 40% coverage). A high rubber coverage is indicative of excellent adhesion between rubber and cord and preserves the reinforced rubber product by preventing possible ply or cord separation.
From such adhesion measurements on known brass-coated steel reinforcing elements it has been found that the adhesion after ageing, i.e. after cured humidity ageing and after steam ageing, will often be considerably reduced and may actually become more than 50% lower than the initial adhesion. The decrease partly depends on the rubber composition used, but mostly on the brass coating and its intrinsic capacity to retain adhesion strength and rubber coverage in ageing conditions.